1. Field of the Invention
The present invention generally relates to systems and methods for a data-driven, closed-loop, hybrid control algorithm for a stencil printing process used in electronics manufacturing and semiconductor packaging.
2. Discussion of the Related Art
The stencil printing process is used in various electronics manufacturing environments. Generally, in relation to electronics manufacturing, stencil printing refers to the use of a stencil as a template for which solder paste or other similar material, including, but not limited to, polymer films, may be applied to an electronic medium such as a printed circuit board (PCB), a semiconductor device or a biological membrane.
The stencil printing process is widely used in surface mounting components to a PCB. Surface mounting, in contrast to thru-hole techniques, refers to a circuit board packaging technique in which the leads or pins on chips and other components are soldered on top of the PCB rather than through the circuit board. Both surface mounting and thru-hole techniques can be applied sequentially to the same PCB. In a typical surface mount PCB manufacturing process, the first step is to “print” solder paste deposits over the metallic contact pads of a PCB. The solder paste deposits are then verified, often by a mere visual inspection. The components are then placed on top of the solder paste deposits, pushing their leads into the paste. When the components have been attached, the solder paste is melted using either a reflow oven, vapor-phase soldering, or an equivalent technology to create the electro-mechanical junctures. Finally, the manufactured PCBs are inspected and tested.
The stencil printing process has also been adapted for use in the wafer bumping process. Wafer bumping is a packaging technology which uses solder bumps to form the interconnect between the integrated circuit (IC) and the package. Similar to the manufacture of PCBs, the process includes the deposition of solder onto each interconnect site.
No matter what use the stencil printing process is applied to, the process and the associated complications with controlling that process are shared. For example, the goal of the stencil printing process when manufacturing of PCBs, as well as when applying solder bumps in the wafer bumping process, is to apply an accurate and repeatable volume of solder paste deposits at precise locations on an electronic medium.
The stencil printing process is characterized by having high process and measurement noise levels and by requiring constant solder-paste-volume deposition at all times. This process has particular characteristics that make it extremely hard to control; some of the most relevant factors are: poorly understood process physics, difficulty in measuring key variables, a high-noise environment, a limited number of measurements, and software and hardware implementation limitations.
The ability to observe the stencil printing process is limited because key variables like solder paste viscosity and hydraulic pressure cannot be directly measured or estimated by most existing industrial production equipment. The process is furthermore corrupted by high degrees of noise caused by inaccuracies in the measurement and by internal system variability. Often, the inaccuracies in the measurement may be disregarded in the case when three-dimensional laser measurement techniques are used. However, the internal system variability has a six-sigma interval of approximately plus or minus thirty percent of the mean of the probability distribution function of the signal, making the process output extremely variable even under constant conditions.
Further, the associated cost of taking a measurement of a system output is high since it is necessary to actually print solder paste deposits on the electronic medium to take the measurements. It should be noted that the measurements are multivariate. The number of such outputs is related to the total number of solder paste deposits printed and inspected. This set of measurements as a whole is considered as a single realization of the system and it is desirable to minimize the number of such evaluations to be able to generate control values while printing as few unusable units as possible.
It has been estimated that between fifty to seventy percent of the total defects in PCB surface mount assembly lines are related to the stencil printing process, and that approximately thirty to fifty percent of the total manufacturing cost is due to test and rework expenses. Thus, the step of stencil printing is considered the most critical in the PCB manufacturing process. Furthermore, a defect that occurs in the early stages of the process will propagate, causing additional rework cost at each step in the process that the PCB goes through without being detected as defective. This stresses the importance of early detection of not only obvious printing errors (e.g., extreme lack or excess of solder paste in a solder brick), but also of possible causes of other defects resulting from degradation of solder paste quality, loss of the working viscosity point, or even machine-related failures. Thus, any attempt to enhance the performance of the PCB manufacturing lines often start with the stencil printing process.
Accordingly, because of the difficulty of controlling the stencil printing process due to the above described limitations, a control system and method of controlling the amount of solder paste deposited to an electronic medium is desired.